What's the difference between "content" and "contented"?

I would suggest that contented is slightly narrower in meaning than content. contented carries the suggestion that the emotion is caused by circumstances, whereas content does not.

The adjective content is complete in itself. It describes a state of being / mind. We can learn to become content with our life, even if it is not the life we would have wished for.

By contrast, the grammatical structure of I am contented suggests that it might be the passive form of X contents me, which implies that some unspecified thing is causing that feeling. One might feel contented by the embrace of a loved one.

My (AusE) intuition is that the "contented" has a warmer, fuzzier feeling associated with it. I'd guess that comes from its use in literature ("They lay down, contentedly in front of the fire."), whereas the shorter form is commonly used in modern newspapers ("The union said they were content with the current arrangement.")

So, I would say "contented" has more of an emotional effect ("pleasantly happy"), while "content" is more neutral ("satisfied"). I have nothing to back this up with. :)